1. Field
Embodiments of the present invention generally relate to electronically configurable and controllable antenna elements. More particularly, embodiments of the present invention relate to the control and configuration of amplitude and/or phase parameters of individual antenna elements such as, for example and without limitation, antenna elements of multi-element antenna arrays, multi-element electronically steerable antennas (MESAs), and the combination of MESAs with multiple-input multiple-output (MIMO) antenna technology.
2. Background
Generally, antennas can be classified into three categories: omni-directional, semi-directional, and highly-directional antennas. These three general antenna categories have different electromagnetic signal directional and gain characteristics (often referred to as “directivity”). Antenna directivity can be defined as the ratio of radiation intensity in the direction of the antenna's peak intensity or the desired direction of operation to the average radiation intensity in all other directions (e.g., total integrated power in all directions captured by the denominator of the ratio which includes the direction of interest). In addition to directivity, antennas are characterized by a radiation pattern, which can be either a two-dimensional or three-dimensional graphical plot of the antenna's signal intensity versus a reference angle.
Omni-directional antennas can have a broad radiation pattern and transmit and receive electromagnetic signals nearly uniformly in all directions. Examples of omni-directional antennas include dipoles, discones, masks, and loops, Semi-directional antennas are capable of focusing desired energy and signals in a desired direction. Examples of semi-directional antennas include patch antennas, panel antennas (both patch and panel antennas are also referred to as “planar antennas”), and Yagi antennas (e.g., a directional antenna having a horizontal conductor with several insulated dipoles parallel to and in the plane of the conductor).
Semi-directional antennas offer improved gain over omni-directional antennas in the desired direction of operation while reducing the gain of and/or potential interference from signals in other directions. As noted above, these characteristics of semi-directional antennas are referred to as directivity. Highly-directional antennas provide a smaller angle of radiation in the desired direction of operation, a more focused beam, and a narrower beam width compared to the above-described general antenna types. Examples of highly-directional antennas include parabolic dish, fixed arrays, and grid antennas (a grid antenna resembles, for example, a rectangular grill of a barbecue with edges slightly curved inward. The spacing of the wires on a grid antenna is determined by the designed operational wavelength of the antenna.).
All three of the above-described general antenna types (i.e., omni-directional, semi-directional, and highly-directional antennas) can also be classified as fixed antenna designs. A fixed antenna design is one that has a fixed gain, a fixed radiation pattern (e.g., fixed directionality), and a fixed direction of operation. An example of a fixed, highly-directional antenna is the parabolic dish antenna, which is commonly used in satellite communications. The parabolic dish antenna includes a reflector that is sized to produce the desired antenna gain and beam width for a specific radiation pattern and can be oriented in the desired direction of operation.
While particularly suitable for fixed gain, fixed location, fixed distance, and fixed direction communication systems, fixed antenna designs are not particularly suitable for applications requiring variable direction and/or variable gain. For example, the gain and radiation pattern of a parabolic dish antenna are fixed based on the size and design of the dish's reflector, and the direction of operation can only be changed by changing the dish's physical orientation. These disadvantages and limitations of static parabolic dish antennas apply to most fixed antenna designs.
An antenna design that offers advantages over the aforementioned limitations of fixed antenna designs is a multi-element electronically steerable antenna (MESA). This type of antenna can be utilized either in a fixed location or in a portable (or mobile) environment. A single MESA can be designed to produce omni-directional, semi-directional, and highly-directional antenna radiation patterns or directivity. The directivity and gain of the MESA are determined by the number of antenna array elements and the ability to determine and control the relative phase shifts and/or amplitudes between antenna array elements.
A MESA can electronically change its gain and radiation pattern (e.g., directivity), as well as its direction of operation, by varying the relative phase shift and/or amplitude of its antenna array elements. Furthermore, a MESA does not require any mechanical components, such as a motor or a servometer, to change its direction of operation, its gain, or its radiation pattern. This allows both its size and weight to be reduced, making the MESA an ideal candidate for portable (or mobile) communication systems. Additionally, because the MESA operational parameters can be modified electronically, the direction of operation of the MESA can be changed more rapidly than a fixed antenna design, making the MESA a good antenna technology to locate, acquire, and track fast moving signals.
Conventional MESA arrays use variable phase shifters (e.g., time delay phase shifters, vector modulators, and digital phase shifters) to control directivity. The input dynamic range and resolution of such phase shifters, however, is limited, which limits the accuracy at which a determined configuration of relative phase shifts can be set. In turn, this limits the accuracy of the resulting beam steering angle of the antenna array and the suitability of the antenna array for certain applications (e.g., high mobility applications). Increasing the number of antenna elements of the array typically allows greater accuracy of beam steering angle but comes with an increased footprint and cost.